1. Technical Field
The present disclosure relates to communication systems, control stations, resource control methods, and microwave base stations.
2. Description of the Related Art
In recent years, in response to a rapid increase in traffic demand, in order to ensure a bandwidth of 1 GHz or more, a small base station that performs communication by using a millimeter-wave band has been introduced. For example, a communication system (which is sometimes referred to as a heterogeneous network) in which a plurality of small base stations are provided in a communication area of a base station that performs communication by using a microwave band is envisioned.
With such a small base station, it is impossible to increase the radio wave travel distance due to the use of the millimeter-wave band, which significantly increases propagation loss.
As one mode that contributes to avoidance of such propagation loss or the enhancement of the communication speed and the expansion of a cell area, there is a scheme in which directivity control (beam forming) is performed in a base station by using a plurality of antenna elements (an antenna array). In the scheme in which directivity control is performed, by directing the radio waves transmitted from the base station in a direction in which a terminal station is present, it is possible to make the radio waves reach a more distant point as compared to a case in which transmission is performed non-directionally and expand a cell area covered thereby. Moreover, since it is possible to improve the signal to interference-plus-noise power ratio (SINR), the terminal station can use a modulation scheme and a code rate with high frequency usage efficiency and therefore perform communication at a high transmission speed (for example, see IEEE 802.15.3c-2009 Standard for Information Technology-Telecommunications and Information Exchange between systems-Local and Metropolitan networks-Specific requirements-Part 15.3: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for High Rate Wireless Personal Area Networks (WPANs): Millimeter-wave based Alternative Physical Layer Extension Amendment is ratified by Standards Board of IEEE).
However, in a stage prior to the establishment of initial connection, since the base station does not have information based on which the direction in which the terminal station is present is determined, the base station cannot transmit a signal by adjusting the directivity so that a signal travels only in the direction in which the terminal station is present.
Thus, a method has been proposed by which a base station divides an area which the base station desires to use as a cell area into sub-areas in accordance with the width of a directivity and transmits a signal while successively switching the directivity such that the directivity is adjusted so that a signal travels toward each of the sub-areas. This method makes it possible to expand a covered area by directivity control while covering the whole of an area which the base station desires to use as a cell area by successive operations.